1. Field of the Invention
This invention generally relates to a signal reproducing apparatus and a computer program which can be used therein. This invention particularly relates to an apparatus for reproducing a signal from a recording medium such as an optical disc, and also a computer program which can be used in the reproducing apparatus.
2. Description of the Related Art
In typical cases, a high-density recording medium such as an optical disc stores address information and user data. It is known to previously record a constant-period signal on a position of an optical disc which precedes the recorded positions of address information and user data. The constant-period signal is reproduced as a continuous wave like a carrier. An apparatus for reproducing information from such an optical disc utilizes a reproduced continuous-wave signal to stabilize an information recovery process. A VFO interval prescribed by the DVD-RAM standards is an example of an interval occupied by a constant-period signal corresponding to a continuous wave. The continuous-wave-corresponding signal has a period equal to, for example, 4T where T denotes a bit period.
A typical information reproducing apparatus contains a PLL (phase locked loop) circuit for recovering a timing signal from a reproduced signal. The PLL circuit generates zero-cross information from the reproduced signal, and generates a phase error signal on the basis of the zero-cross information. The PLL circuit locks up in response to the phase error signal. Various factors cause asymmetry in the waveform of the reproduced signal. The waveform asymmetry tends to cause the PLL circuit to generate a wrong phase error signal in response to a reproduced continuous-wave signal. The wrong phase error signal makes it difficult for the PLL circuit to lock on a correct frequency and a correct phase during the reproduction of address information and user data which follow the continuous-wave-corresponding signal.
Also, it is known to previously record repetition of a specified pattern on a position of an optical disc which precedes the recorded positions of address information and user data. The repetition of the specified pattern is used instead of the continuous-wave-corresponding signal.